1. Field
The present invention relates in general to magnetic field and current sensors, and more particularly, without limitation, to signal processing for magneto-resistive sensors.
2. Related Art
Magnetic field sensors have applications in magnetic compassing, ferrous metal detection, and current sensing. They may detect magnetic field variations in machine components, the earth's magnetic fields, underground minerals, or electrical devices and lines.
In magnetic sensors, specifically Anisotropic Magneto-Resistive (AMR) bridge sensors, a thin film of magneto-resistive material is placed on a silicon substrate to precisely measure the intensity and/or direction of local magnetic fields. Since the deposition of thin films on silicon substrates can utilize the processes of a semiconductor foundry for fabrication, further steps to create adjacent semiconductor circuit elements can be added. These semiconductor circuit elements traditionally have not been co-located on the same substrate as magneto-resistive sensors due to the incompatibility of sensor thin-films with traditional semiconductor manufacturing processes.
Typically, magneto-resistive sensors use Permalloy, a ferromagnetic alloy containing nickel and iron, as the magneto-resistive material. Often, the Permalloy is arranged in thin strips of Permalloy film. When a current is run through an individual strip, the magnetization direction of the strip may form an angle with the direction of current flow. As the magnetization direction changes, the effective resistance of the strip changes. Particularly, a magnetization direction parallel to the current flow direction results in maximum resistance through the strip and a magnetization direction perpendicular to the current flow direction results in minimum resistance through the strip. This changed resistance may cause a change in voltage drop across the strip when a current is run through the strip. This change in voltage may be measured as an indication of change in the magnetization direction of external magnetic fields acting on the strip.
To form the magnetic field sensing structure of a magneto-resistive sensor, several Permalloy strips may be electrically connected together. The Permalloy strips may be placed on the substrate of the magneto-resistive sensor as a continuous resistor in a “herringbone” pattern or as a linear strip of magneto-resistive material, with conductors across the strip at an angle of 45 degrees to the long axis of the strip. This latter configuration is known as “barber-pole biasing.” It may force the current in a strip to flow at a 45-degree angle to the long axis of the strip, because of the configuration of the conductors. These sensing structure designs are discussed in U.S. Pat. No. 4,847,584, Jul. 11, 1989, to Bharat B. Pant and assigned to the same assignee as the current application. U.S. Pat. No. 4,847,584 is hereby fully incorporated by reference. Additional patents and patent applications describing magnetic sensor technologies are set forth below, in conjunction with the discussion of FIG. 2.
Magnetic sensors often include a number of re-orientation elements or “straps” through which current may be run, for controlling and adjusting the sensing characteristics. For example, magnetic sensor designs often include set/reset and/or offset re-orientation elements or “straps” (hereinafter “set/reset straps” and “offset straps”).
Offset straps serve to cancel or correct for external magnetic fields. Set/reset straps help to re-orient the magneto-resistive thin film grain structure for best measurement accuracy. This process of re-orienting magneto-resistive films utilizes the set/reset strap metallization to apply brief, intense magnetic field strengths to force arbitrarily orientated thin film grains substantially into a single direction. This brief field application “sets” the film into one orientation. A second brief field application in a similarly intense but opposite direction “resets” the film's grain orientation. Repeated set and/or reset fields are used to ensure film granules remain undisturbed and in a relatively known magnetic orientation.
While the set/reset straps themselves have typically been located on-chip, driver circuitry for these straps has typically been located off-chip, resulting in space inefficiencies. Off-chip solutions typically pulse a current through one or more straps (typically metal) over the magneto-resistive sensor bridge, but external board-level circuitry is used to switch and generate the current pulse.
Similarly, other components, such as operational amplifiers, transistors, capacitors, etc., have typically been implemented on a separate chip from the magnetic sensor. For example, signal conditioning and electrostatic discharge circuitry is typically located off-chip. While this may be fine for some applications, for others, in which physical space is at a premium, it would be desirable to have one or more of these semiconductor components as part of the same chip as the magnetic sensor. Thus, a single-chip design, and in particular, a design having set/reset driver circuitry located on-chip, would be desirable.